An enormous amount of organic waste is produced daily, by the human food chain, in agriculture and by the agro-industrial sector. This organic waste accounts for a large part of solid urban waste, it is one of the main components of sewage and constitutes the residue of the agro-industry and other industries.
The problem is how to treat this waste, so as to prevent its entering the sewerage, and to reutilize it.
Aerobic decomposition by means of micro-organism (composting) is undoubtedly a very efficient and economic technique. The biological treatment plants currently known generally operate by feeding the organic matter, either continuously or discontinuously, into treatment tanks of various shapes, where it is stirred by mechanical means, so as to achieve a state of homogenization, and moved forward, so as to make room for the inflowing waste matter; furthermore, the treated matter is kept well aerated, so as to ensure that the micro-organism which perform the biological process, are kept well supplied with the necessary amount of oxygen.
Various waste treatment plants are known which operate according to the above mentioned technique, however, they all feature drawbacks which limit their efficiency. For instance, Swedish patent 7306001-4, deposited on Apr. 27, 1974 by Johnson Construction Company AB, provides for a treatment tank with a sloping base, above which are suspended one or more sets of screw propellers, pivoted around a horizontal axis attached to a roller bridge, which are dipped inside the mass and move across the entire length of the treatment tank, from the inflow will to the outflow wall, thus mixing and pushing the mass forward to make space for new waste matter. When the screw propellers reach the outflow wall they are turned around their hinges, pulled out of the mass and then moved back to the inflow wall, where they are once again dipped into the mass and begin a new cycle. The air for the micro-organisms performing the biological process is pumped through the screw propellers themselves, which are hollow inside. This kind of system features various serious drawbacks, such as a loss of about 50% of the working time for the idle return of the screw propellers across the treatment tank after each process cycle, the non-uniform progress of the mass from the inflow side of the treatment tank to the outflow, due to the necessary extraction and re-immersion of the screw propellers inside the mass, and the prerequisite of a treatment tank with a heavily sloping base, to help the mass in its progress, since otherwise it would exercise too strong a resistance against the screw propellers, jeopardising the correct operation of the system's mechanical parts. This loss of time translates into a reduction of the treating capacity, while the non-uniform progress of the mass, as well as the variation, in the order of 50%, of the time required by the waste matter to achieve a sufficient degree of maturation, may completely alter the outcome, which is measured as the degree of maturation and is closely linked to the time spent in the treatment tank by the totality of particles composing the mass, and other parameters, such as mixing and aeration.
Italian patent 23342 of Oct. 18, 1983, deposited by Secit SpA, provides for a technique similar to the one described in the Swedish patent, whereby the screw propellers advance in the same direction as the mass, while in this case the screw propellers are removed from the mass by means of translation along their axis, perpendicularly to the base of the tank, and the aeration of the mass takes place through protected pipes laid on the bottom of the treatment tank.
With the exception of the non-uniform advance of the mass, which defect is remedied by the extraction and re-immersion of the screw propellers, which translate along their axes, no remedy is found for the other drawbacks of the Swedish patent, on the contrary, to these one must add the further disadvantage of having to provide for a building large enough to house the entire plant, which is quite tall, so as to enable the lifting of the screw propellers.
Since all plants of this kind must be adequately sealed from the outside, for obvious reasons of environmental impact, a higher building, besides entailing higher construction costs, also requires higher operating costs, due to the larger amounts of air to be circulated within the building and then deodorized, since the number of change of air per hour must remain equal.
Italian patent of Aug. 7, 1987 in the name of Sorain Cecchini SpA describes a technique whereby the organic matter is contained inside a preferably sloped treatment tank and one or more sets of slanted screw propellers, suspended from a sliding bridge parallel to the tank base, stir and move the mass, not by pushing it but by pulling it toward the outflow, in order to reduce the stress exercised by the mass on the screw propellers.
In this case, the screw propellers move in an opposite direction compared to the mass, but this plant too features the drawback of their idle return run, as well as the overturning of the screw propellers at the end of the cycle and their re-immersion into the mass, thus featuring the same drawbacks as the previous invention.